The design of a water based thermal store for use in a domesticcentral heating system has been investigated theoretically,experimentally and numerically. The transient operation of thestore during both the space heating and domestic hot-watermodes of operation have been investigated separately.Heat transfer correlations in terms of Nusselt and Rayleighnumbers have been developed in order to predict the naturalconvection heat transfer coefficient for the outside surface ofthe horizontal axis finned tube heat exchanger coil locatedwithin the store. These heat-transfer correlations can predictthe value of the heat transfer coefficient with an accuracy ofbetter than 5% and are in good agreement with existing heattransfer correlations developed for the same geometry of finnedtubes and modes of heat transfer. The effect of the water flowrate in the heat exchanger coil on the internal heat transfercoefficient is also investigated. This flow rate should beabove 4 litre/minute to achieve a high rate of heat-transferfrom the wall of the heat exchanger to the water in the pipe.A detailed investigation of the use of horizontal and verticalbaffles to increase the effectiveness of heat delivery in thedomestic hot water mode has been carried out. Some improvementscan be achieved by the use of a horizontal flat plate locatedin the middle of the store. This plate, when correctly sizedenhances stratification and hence improves the effectiveness ofheat recovery. Vertical plate arrangements and a rectangularduct situated around the upper heat exchanger coil were foundto be ineffective. However, due to an increased velocity of thewater around the heat exchanger, the external heat transfercoefficient of the heat exchanger was increased by 12%.The comparison of experimental observations with computersimulations of the development of the thermocline in the storeduring the space heating mode of operation showed the presenceof a jet in the bottom region of the store at the return inlet.The jet induces a significant amount of mixing in the storewhich reduces the effectiveness of heat recovery. Correlationsin terms of Richardson number and effectiveness of heatdelivery have been developed to characterize the effect of thisjet. An inlet arrangement designed to achieve a Richardsonnumber exceeding 3 significantly reduces the mixing created bythe jet and can increase the amount of heat delivered in thespace heating mode by approximately 5%.
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